The American Petroleum Institute (API) has classified various independent screwed wellheads that are well known in the art for securing a surface casing, and for supporting various well servicing equipment. Independent screwed wellheads support independently secured heads for each tubing string supported in a well bore. Independent screwed wellheads are widely used for production from low-pressure production zones because they are economical to construct and maintain.
It is well known in the art that low pressure wells frequently require some form of stimulation to improve or sustain production. Such stimulation procedures typically involve pumping high pressure fluids down the casing in order to fracture production zones. The high pressure fluids are often laden with proppants, such as bauxite and/or sharp sand.
FIG. 1 illustrates a prior art Larkin style independent screwed wellhead apparatus. The independent screwed wellhead apparatus includes a casing mandrel 20 supported in a casing bowl 22 of a wellhead 24 by a lockdown nut 26 that threadedly engages pin threads on an exterior periphery 37 of the wellhead 24. In the Larkin-style wellhead the casing mandrel 20 extends above the lockdown nut 26. The wellhead 24 is secured to a surface casing 28 that forms an outer periphery of the well bore at the surface. The casing mandrel 20 is supported in the casing bowl 22, and snubbed by the lockdown nut 26. The casing mandrel 20 supports a production casing 30 within the wellbore. The production casing 30 is threadedly connected to the casing mandrel 20 by bottom box threads 32 that engage threads 34 on the outer periphery of the production casing 30. A full-bore axial passage 36 extends through the casing mandrel 20 concentric with the bottom box threads 32. Top box threads 38 can be used for connection of an adapter that permits connection of a well stimulation tool. A fluid seal is provided between the casing mandrel 20 and the casing bowl 22 by annular grooves 40 that retain O-ring seals.
FIG. 2 schematically illustrates a cross-sectional view of another prior art independent screwed wellhead apparatus of a known configuration that is commercially available from Wellhead Inc. of Bakersfield, Calif., USA. In FIG. 2, neither the production casing nor the adapter for the well stimulation tool is shown. Accordingly, the top 38 and bottom 32 box threads can be seen. The casing mandrel 20′ has a lower profile, and therefore has a shorter axial passage 36′. The remainder of the casing mandrel 20′ is substantially the same as corresponding parts of the casing mandrel 20 illustrated in FIG. 1, except that a top surface of the lockdown nut 26 is horizontally aligned with a top surface of the casing mandrel 20′ shown in FIG. 2.
FIG. 3 schematically illustrates the casing mandrel 20′ shown in FIG. 2, in a typical configuration used for prior art well stimulation procedures. The casing mandrel 20′ is threadedly connected to the production casing 30, and to a flanged casing pin adapter 42, and is secured to the wellhead 24 using lockdown nut 26. The flanged casing pin adapter 42 is typical of those in use today, in that the sole means for coupling the pin adapter 42 to the wellhead 24 is a pin thread 44 that engages the top box threads 38 of the casing mandrel 20′.
The flanged casing pin adapter 42, includes a body that forms an axial passage 46 with a cylindrical section 46a and an upward widening truncated conical section 46b. The function of the flanged casing pin adapter 42 is to permit connection of well stimulation tools and other equipment (e.g. a high pressure valve or a blowout preventer (BOP)) to the casing mandrel 20′. Accordingly the flanged casing pin adapter 42 has a flanged top surface 48 that enables secure connection of any flanged component. An annular groove 50 accommodates a flange gasket for preventing fluid leakage across the interface between the flanged casing pin adapter 42 and the other component.
In a typical well stimulation procedure, a casing saver (not shown), such as a casing packer as described in U.S. Pat. No. 4,993,488, which issued to Macleod on Feb. 19, 1991, is inserted through a BOP and into the production casing 30. The casing saver is sealed off against the production casing 30 and high pressure fluids are injected through the casing saver into a formation of the well. While the casing saver protects the exposed top end of the production casing 30 from “washout”, it does not relieve the top box thread 38 or the pin thread 44 from mechanical stress induced by the elevated fluid pressures generated by the injection of high pressure fracturing fluid into the well. In a typical fracturing operation, high pressure fluids are pumped into the well at around 9500 lbs per square inch (PSI). If “energized fluids” or high pumping rates at more than 50 barrels per minute are used, peak pressures can exceed 9500 PSI. In general, the threads retaining the flanged casing pin adaptor 42 in the casing mandrel 20 are engineered to withstand 7000 PSI, or less. Consequently, high pressure stimulation using standard equipment can expose the flanged casing pin adaptor 42 to an upward pressure that exceeds the strength of the bottom pin thread 44. If either the top box thread 38 or the pin thread 44 fails, the flanged casing pin adaptor 42 and any connected equipment may be ejected from the well and hydrocarbons, and stimulation fluids may be released into the atmosphere. This is potentially dangerous and an undesirable situation.
Furthermore, use of a casing saver to perform well completion or re-completion slows down operations in a multi-zone well because the flow rates are hampered by the reduced internal diameter of the casing saver. Moreover, the casing saver must be removed from the well each time the fracturing of a zone is performed, in order to permit isolation plugs or packers to be set, as it is necessary to isolate a next zone to be stimulated. It is well known in the art that the disconnection of fracturing lines and the removal of a casing saver is a time consuming operation that keeps expensive fracturing equipment and/or wireline equipment and crews sitting idle. It is therefore desirable to provide full-bore access to the well casing in order to ensure that transitions between zones in a multi-stage fracturing process are accomplished as quickly as possible.
Applicants have designed a wellhead that overcomes these problems by providing an improved casing mandrel for securing components to an independent screwed wellhead. The improved casing mandrel is described in co-pending United States Patent Application Publication No. 20040231856 entitled CASING MANDREL WITH WELL STIMULATION TOOL AND TUBING HEAD SPOOL FOR USE WITH THE CASING MANDREL, which was filed on May 19, 2003, the specification of which is incorporated herein by reference. However, the independent screwed wellheads such as the Larkin and Wellhead Inc. styles described above, which remain in wide use do not accommodate secure connection of high pressure components for reasons described above.
There therefore exists a need for adapters that provide full-bore access to a casing in a well to be stimulated, while significantly improving safety for well stimulation crews by ensuring that a hold strength of the adapter through which well stimulation fluids are injected exceeds fluid injection pressures by an adequate margin to ensure safety.